Thyratron type microwave switching apparatus



THYRATRON TYPE MICROWAVE SWITCHING APPARATUS Filed July 20, 1964 3 Sheets-Sheet l CHARGING DISCHARGING CIRCUIT I CIRCUIT j Fig.2.

BLOCK DIAGRAM OF CHARGING AND DISCHARGING CIRCUITS E CHARGING PFN,-r REACTOR HOLDOFF gx 30o COAXIAL CABLE DIODE TRANSFORM- 0 I2 ANODE LoAD 23 i 2q 24 -fl\- son TRIGGER R P (4 POUT o- FILTER 2 H INjlf CIRCUIT 40619 CATHODE WINDOWS lf E WAVEGUIDE I AND CONTROL 26 27 GRID IQ SPOKE IIOV-60- FILAMENT VOLTAGE RESISTOR To MEASURE CURRENT IIOV-60- RESERVOIR VOLTAGE 2s VARIACS May 30, 1967 H. GOLDIE 3,323,003

THYRATRON TYPE MICROWAVE SWITCHING APPARATUS Filed July 20, 1964 5 Sheets-Sheet, 2

Circular Ceramic body Molybdenum Plate Cathode baffle R ectangula waveguide AND CONTROL GRID Heat 55 Shield thode Heater 65 6 l-I e1iarc LEAD nngs RfjfkbO/R Trlple 1H5, HEAR-R1090 Heharc pressure pM/Cw 0F; envelope ld window Fig.3

Filed July 20, 1964 May 30, 1967 H. (501.015 3,323,003

THYRATRON TYPE MICROWAVE SWITCHING APPARATUS 3 Sheets-Sheet 5 /CERAMIC COVER 47 j 5 0 COPPER ANODE CUP CIRCULAR CERAMIC ATWENVELOPE L@/I NNER HELIARC RING Q QUTER HELIARC RING MOLYBDENUM PLATE TRIPLE-IRIS MICROWAVE WINDOWS 58 CATHODE HEATER SHIELD AND CATHODE BAFFLE 56 WCATHODE 61 REsE Ryan; 5? HEADER United States Patent Ofiice 3,323,963 Patented May 30, 1967 3,323,003 THYRATRON TYPE MICROWAVE SWITCHING APPARATUS Harry Goldie, Randallstown, Md., assignor, by mesne assignments, to the United States of America Filed July 20, 1964, Ser. No. 383,639 1 Claim. (Cl. 315-39) This invention relates to microwave switching apparatus and more particularly to a highpower microwavedischarge switch of the thyratron type.

The trend in present day microwave practice is toward higher power requirements in transmission systems and in the components of said systems, including microwave switches. There are several ways to switch microwaves, such as ferrite Faraday rotators, devices that utilize the B-H loop of ferrites, high vacuum cavities employing the multipactor effect, and low pressure gaseous devices.

The principal problem involved with the prior art devices has been found to be that they require switching times on the order of one-half a microsecond for low gas pressure microwave switches up to about milliseconds for mechanical microwave switches. Such prior art devices are generally incapable of handling microwave power requirements greater than 50 kilowatts.

Prior art devices of the gaseous pressure microwave switch type have also been further characterized in that they have a relatively short tube life due to such factors as cathode poisoning, gas cleanup and sputtering.

A primary object of this invention is to provide a highpower microwave switch of the low gas pressure type which iscapable of firing in less than 40 nanoseconds, is capable of handling microwave powers on the order of a few hundred watts to and aboue 1 megawatt and is capable of eliminating gas cleanup and maintaining a constant gas pressure in the switch.

Another object of thi invention is the provision of a fast-acting, low-loss gas discharge microwave switch.

A general object of the invention is to provide an improved thyratron type microwave switch having the following general characteristics:

Recovery time to 30 microseconds. VSWR (over entire C-band) 1.2.

Pulse repetition rate up to 500 pulses per second.

Band width equal to full-wave guide band width.

The manner by which the above objects are achieved will be more fully understood from a reading of the following description of a preferred embodiment of the invention, reference being had to the accompanying drawing in which:

FIGURE 1 is a schematic diagram of the circuitry necessary to supply voltages, currents and triggering potentials to fire the switch;

FIGURE 2 is a block diagram of the supply networks necessary to supply voltages, currents, and triggering potentials to operate the switch;

FIGURE 3 is a cross sectional view of the assembled switch; and

FIGURE 4 is an exploded view of the switch.

The present invention utilizes a section of a wave guide in which a pulsed D.C. arc discharge is generated. During discharge an arc plasma of high electron density is established transversely of the direction of propagation of microwaves to block the passage of said microwave signals. In the absence of the arc plasma, the switch acts as a low loss section of wave guide. The switch is fired by the application of a triggering potential to a dual control grid built into and as part of the section of the wave guide. Essentially the gas thyratron microwave switch is a duogrid thyratron.

Referring now to FIGURES 1 and 2, switch 10 is an assembly comprising an anode 12 connected to a power supply 15 through an isolating network 16 and a pulse forming network 17, a grounded cathode 11, and the dual grids 13 and 14 which are mechanically parts of and aligned transversely of the broad walls of the switch 10. The grids 13 and 14 are biased through a resistor 20 by a grid supply 21, and are coupled through a condenser 22 to a grid filter circuit 23 which is in turn coupled to a source of trigger pulses 24. The grid filter circuit is utilized to de-spike grid potentials.

Referring to FIGURE 1, in the absence of a pulse from the trigger on the switch 10, the pulse forming network will charge from the supply 15 through an isolating element 16 and the load 19. The pulse forming network shown in the preferred embodiment of this invention is a section of a SO-ohm cable cut to a length which gives a two-way delay time of one microsecond.

With a trigger on the grid of the switch 10, the tube essentially acts as a short and will discharge the pulseforming network. An isolating element, the hold-off diode 25, has been included to insure that discharge takes place in the tube. In that the delay time of the pulse forming network is one microsecond, the time between the formation of the conducting path through the microwave switch and the termination of the complete discharge of the network is one microsecond.

Filament voltage is supplied from a variac 26 operating off of an AC. line. Heater voltage for the hydrogen reservoir 27 is supplied from a second variac 28.

In operation a potential is applied to the anode 12 by the power supply 15 through the network 17, and the grids 13 and 14 are biased below cut oif and no current flows in the tube. With no current flowing, the tube is open and signals received by the switch are passed through it. The switch may be fired either by a triggering pulse before a microwave pulse is received at one of the microwave permeable windows 29 and 29a or during the period when a microwave pulse is being received by said window or after a microwave pulse is received at said windows. Upon the application of the triggering pulse from a source of said pulses 24, the bias of the switch tube would be raised above cut-off and a current flow will be established between the plate 12 and cathode 11 of the tube in 30 nanosceonds. This current will have built up to the full rate.

for emission of the cathode within this period. This heavy flow of current instantaneously ionizes the gas so that the combined heavy electron and ion density of the discharge plasma thus established entirely prevents the transmission of microwave energy and said energy must then be reflected. The electron current established by the triggering pulse flows until the charge on the pulse forming networks 17 has been dissipated which occurs in one microsecond due to the choice of pulse-forming network.

The electron density is the combination of the electron gas in the plasma and the charge of the pulse forming network therefor after the termination of the current impulse the electron density is only a function of the decay time of the plasma. De-ionization which takes place in approximately 20 to 30 microseconds, is insensitive to variations in pressure and to potentials and is primarily dependent on tube geometry. The de-ionization time for the preferred embodiment of this invention may be re- 3 duced by the application of pulse sweeping fields for the rapid removal of electrons.

A preferred embodiment of the mechanical structure of the switch tube is shown in FIGURES 3 and 4 as a short section of standard rectangular oxygen-free copper wave guide 30. The use of oxygen-free material is essential in gaseous discharge devices since heating of the copper will cause oxygen molecules to be liberated from the surfaces and contaminate the hydrogen. The OFHC copper is also necessary for brazing operations in an inert or hydrogen atmosphere.

The grids 31 and 32 are made by countersinking in each of the broadwalls 33 and 34 to a fixed depth for the dual purpose of providing a support for the cathode and anode assemblies and for the purpose of making the metal part of the grid as thin as possible with due allowance for mechanical strength.

In the preferred embodiment of this invention the metal grid was made with a thickness of 0.040 inch. A plurality of holes 35 and 36 were formed through the metal to complete the grid structure. In the preferred embodiment there were fourteen holes per grid, each hole having a diameter of 0.125 inch.

The grids 31 and 32 are centrally aligned and spaced apart by the distance between the top and bottom walls 33 and 34.

The opposite ends of section are sealed by end Walls 37 and 38 which have centrally disposed microwave permeable windows 39 and 40, respectively.

In the preferred embodiment of this invention, the microwave permeable windows have been constructed out of kovar and 707 glass and are of triple-iris design.

The anode assembly 41 comprises an outer heliarc ring 42 of kovar attached concentric to the grid 32 within the circular bore 43 by brazing. A ceramic envelope 45 attached to an inner heliarc ring 44 by brazing acts as a support for the copper anode cup 46. In the preferred embodiment of this invention a molybdenum plate 47 has been bonded to the target side of the anode cup. A molybdenum plate was chosen in that it has the desirable ability of being able to resist the deteriorating effects of ion bombardment. The anode cup 46 is held rigidly in place by ceramic cover 47 by brazing the ceramic cover to the circular flange 48 where the flange 48 is brazed to the ceramic envelope 45. An anode lead may be attached to the drilled receptacle 49 located in the pedestal 50 of the anode cup. The inner heliarc ring is concentric with the outer heliarc ring and is bonded to the outer ring by a heliarc weld.

The spacing between the target of the anode and the grid 32 will be dictated by the maximum voltage that the pulse forming network can handle. In the preferred embodiment this spacing was 0.100 inch.

The spacing between the anode target 47 and the grid 32 is crucial in that when the switch is fired electrons will be built up due to the breakdown of the grid cathode space leading to the growth of a plasma near the grid apertures. These electrons will be accelerated in the anode space causing ionization. This ionization leads to a cumulative modification of the original potential distribution resulting in anode breakdown and establishment of a conducting channel through the apertures and 36 of the grid. Thus, it will be obvious to those skilled in the art that the configuration of the electric field existing in the area of the anode will have a fundamental effect on the switching time of the tube.

The spacing between the circumferential edge of the target 47 and any other protrusion, such as the ceramic envelope 45 i also critical and should be minimized to prevent arcing. In the preferred embodiment this distance was 0.010 inch.

The cathode and reservoir support assembly 51 is attached to the lower broad wall 34 in much the same manner that the anode assembly is attached to the upper broad wall. An outer heliarc ring 52 made out of kovar is mounted in the circular bore 53 by brazing and is welded to an inner heliarc ring 54 where said inner heliarc ring supports the cathode assembly. A cylindrical ceramic envelope 55 acts as a support for the cathode 60, cathode heater shield 56, the bafile 58, and the hydrogen reservoir 61 in conjunction with a header 62 which has been welded to the envelope. The ceramic envelope is bonded to the heliarc ring 54.

Ceramic envelopes 45 and 47 in the anode and cathode assemblies respectively have been employed in that this material adsorbs less hydrogen gas on the surface as compared to other envelope materials. Ceramics also have a much higher thermal conductivity thus allowing for greater heat dissipation and operation at higher temperatures.

In the preferred embodiment of this invention, an all ceramic-metal construction is employed in that this construction allows for a maximum of control in the machine tolerances of parts and further allows for a simple, rugged switching device.

The cathode 60, its heater 60a and heat shield 56 are supported by a hollow cylinder 57 having an annular inwardly extending flange 57a by brazing the heat shield to the flange. The cathode 60 is of the concentric circular vane type and is supported by its heater 60a.

A baffle plate 58 is disposed in longitudinal alignment with and above the cathode. The baffle is supported by the heat shield 56 by radially extending fingers attached to the heat shield.

A source of hydrogen gas 61 is supported by 62 through the springs 59 and 590.

External heater leads for the cathode and hydrogen reservoir are provided at 65 and 66, respectively.

Ideally, by supplying additional gas to the switch tube to replace cleanup losses the hydrogen reservoir should maintain constant equilibrium pressure within the tube. This ideal is approached in practice by employing a reservoir comprising a heated compound of hydrogen in an exothermic hydrogen occluder. In the preferred embodiment of this invention the occluder is titanium forming a metallic hydride. This reservoir has the advantage of eliminating gas cleanup, allowing manual pressure adjustments for optimum microwave operation and stabilizing gas pressure at any setting within the operating range of the reservoir. This source of free gas ions is utilized to replace cleanup losses and stabilize pressure. With the introduction of this reservoir into the tube, tube pressure can be stabilized and maintained by simply setting the reservoir heater voltage for optimum switch operation. It will be obvious to those skilled in the art that the use of this reservoir will greatly extend the life of the tube.

The distance between the lower grid 31 and the cathodebaflie is not critical and in the preferred embodiment of the invention is 0.900 inch. This spacing should not be too close in that abnormally high grid potentials would be necessary to initiate a plasma in the lower grid cathode region, whereas, an increase in spacing would merely make the tube physically large.

The input lead for the grids may be attached to any portion of the microwave section proper. In the preferred embodiment the lead for trigger potentials was attached to the bottom wall 34 of the tube. The rest of the circuitry must be suitably insulated from the trigger pulse. a

The essential requirement of the switch tube is to establish an electron density in the wave guide independently of the RF field. It must be of sufiicient magnitude to reflect almost all of the incident microwave power and to absorb very little. The creation of this electron density must be accomplished in a time interval much shorter than the RF pulse width normally employed in microwave systems. The DC. voltage control switch tube 30depicted in FIGURE 1 is essentially a gas-filled thyratron in which the two grids serve the dual role of guiding the electromagnetic waves and serving as the the header control grids of the triode. The medium which physically blocks the flow of microwave energy through the switch, when it is fired, is the gas discharge plasma, that is, the portion of the discharge comprising the positive gas ions and electrons within the microwave section 30. This plasma is formed after a triggering signal has been applied to the dual grids 31 and 32 thus causing grid current to flow. When this current reaches a critical value, conduction is transferred to the anode and the commutation intervals start. A plasma forms and acts as the barn'er to the transmission of microwave energy.

The actuating time of the switch is an insensitive function of pressure except at the extreme operating limits of the switch.

The switching time is a sensitive function of anode potential. The reason for the actuating time being a dependent function of the static anode potential is that the electrons ejected into the wave guide space by the positive trigger pulse come under the influence of the anode field lines. These electrons attain high kinetic energies rapidly traversing the wave guide toward the anode where a plasma begins to form. This plasma is highly conductive and extends the anode toward the cathode resulting in a plasma filling the wave guide in a very short time. The nominal switching time is approximately 30 nanoseconds.

The switch tube shown in FIGURE 1 and having the following characteristics has been built and successfully operated:

Anode voltage 2 kilovolts.

Grid trigger 100 volts.

Gas hydrogen.

Pressure approximately 0.2 millimeter of Hg.

Power switched at one micro second pulse length low level to greater than one megawatt A high-power microwave switch of the type with which this invention is concerned has several applications. It may be used in laboratory switching systems, pulse divisions schemes of the transmit-receive type, and in power enhancement pulse compression devices whereby onehalf of the divided pulse is delayed and combined with the other half into a pulse is delayed and combined with the other half into a pulse of half the original width and twice the peak power. The absence of spike leakage in the switch eliminates the need for crystal protectors in monopulse radar. This switch has particular application, because of the extremely low cold insertion losses, as a discharge device in ring resonators.

Changes, modifications and improvements to the abovedescri'bed embodiments in my invention may be made by those skilled in the art without departing from the precepts of the invention. The appended claim defines the features of novelty of the invention.

What is claimed is:

A gaseous pressure microwave switch comprising a section of rectangular wave guide having two pairs of opposed parallel walls and adapted to propagate electromagnetic waves, one pair of said walls having a plurality of holes formed therein in circular patterns and in an aligned opposed relationship, an external source of electrons disposed opposite the openings in the first wall of said walls and in aligned relationship with the openings in said pair of walls, a bafiie member located between said source of electrons and the openings in the first wall of said pair of walls and in aligned relationship with the openings in said pair of walls, a first ceramic envelope for housing said external source of electrons and said bafile member, means for attaching one end of said first envelope to said first wall to form an external appendage to said wave guide that preserves the pressure within said microwave switch, an external receiver of electrons disposed opposite the openings in the second wall of said walls and in aligned relationship with the openings in said pair of walls, a plate of ion bombardment resistant conductive material mechanically afiixed to said receiver of electrons located between said receiver of electrons and the openings in said second wall of said pair of walls and in aligned relationship with the openings in said pair of walls, a second ceramic envelope for housing said external receiver of electrons and said plate, means for attaching one end of said second envelope to said second wall to form an external appendage to said wave guide that preserves the pressure within said microwave switch, and a continuously heated source of gas ions located externally of said rectangular wave guide within said first envelope.

References Cited UNITED STATES PATENTS 2,826,718 3/1958 Larson et al. 333l3 X 2,919,368 12/1959 Goldberg et al 313-178 2,948,825 8/1960 Riley et al. 3l3196 X 3,023,380 2/1962 Hill 333-13 JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner. 

